Power now, efficiency after

Science · Energy · ages 17-18

Name ______________________ Date ____________

What is one watt?
- One joule per second
- One newton per metre
- One joule per newton
A cyclist holds 8 metres per second against 30 newtons of drag. What power in watts is needed?
Answer: ______________
What is efficiency?
- Total input over useful output
- Useful output over total input
- Wasted energy over useful output
You know a car's thrust and its top speed. Which power form fits?
- Force times velocity
- Work over time
- Efficiency ratio
An engine takes in 2000 J and puts out 500 J of useful work. Give the efficiency as a ratio.
Answer: ______________
An engine takes in 2000 J and gives 500 J of useful work. Where did the rest go?
- Destroyed, energy is lost
- Stored in the fuel tank
- Wasted as heat and sound
A leaflet claims a motor with efficiency 1.2. What is the verdict?
- Possible for new designs
- Impossible, output cannot beat input
- True at low speeds only
A car cruises at 25 metres per second against 400 newtons of drag. What engine power in watts holds it there?
Answer: ______________

Answer key

For grown-ups. Fold this page away before handing over the rest.

One joule per second · A watt rates work per time, and one joule each second is one watt.
240 · At steady speed, power is drag times speed: 30 times 8 is 240 watts.
Useful output over total input · Efficiency scores the useful share of what you paid in, so it sits below one.
Force times velocity · Speed plus thrust is a snapshot, and snapshots want force times velocity.
0.25 · Divide useful by total: 500 over 2000 is 0.25.
Wasted as heat and sound · The missing share is not destroyed but scattered, mostly as heat and sound.
Impossible, output cannot beat input · Useful output never beats total input, so any ratio above one is impossible.
10000 · Steady speed means drag times speed: 400 times 25 is 10000 watts.